A company that measures internet service reliability has given Microsoft the top score in a test of operating system update services.

Microsoft's Windows Update was available 100 percent of the second quarter of 2008, Pingdom said in a blog posting on Friday. Apple's service was down two hours and 34 minutes, with 99.9 percent uptime, and Canonical's Ubuntu version of Linux was down one day, five hours and 45 minutes, for 98.64 percent uptime.

"Microsoft wins this one hands down," Pingdom said. It noted that Ubuntu's service also is available through mirror sites, however.

The company tested the three services every five minutes.

During the second quarter of 2008, Canonical's update service for the Ubuntu operating system had more downtime than Apple's or Microsoft's services.